*2.2. Film Colour and Opacity*

The nutritional and flavour protection of food once exposed to visible and ultraviolet light are both determined by the optical properties of food packaging films [33,34]. These properties also impact how well the film is tolerated by a user. Colour parameters, whiteness index (WI), and opacity measurements were used to assess the prepared gel films' optical properties. Compared with the control gel film (Ch-0% EPPE), Ch-EPPE films showed significantly (*p* < 0.05) lower levels of L\*. This demonstrated that the darkness intensity of Ch-EPPE gel films increases as their EPPE content increases. The films were darker as a result of light scattering and refraction produced by phenolic EPPE components [35]. Ch-5% EPPE film showed a maximum 11.94% reduction in lightness compared to the control film (Ch-0% EPPE).

Chitosan gel films' b\* (yellowness/blueness) and a (redness/greenness) values were considerably (*p* < 0.05) impacted by the addition of EPPE. As the EPPE concentrations climbed from 0 to 5%, the gel films' b\* values increased from (1.03 ± 0.01) to (1.98 ± 0.04) (an indication of the trend towards yellowness) and their a\* values from 0.84 ± 0.01 to 1.12 ± 0.03 (an indication of the trend towards redness).

Based on these results, the existence of phenolic components and coloured compounds inside the integrated EPPE as well as the inner structure formed during film drying might be responsible for the addition of EPPE's remarkable effect on the colour of the resultant Ch-EPPE gel films [35,36]. Similar colour measurement findings were achieved when green tea extract and CH were combined, producing films that were a deeper shade of greenish yellow [29]. The films' level of whiteness and opacity were determined using opacity tests and a WI estimate. The WI of the Ch-EPPE was substantially different (*p* < 0.05) from that of the pure CH- film, as indicated in Table 2.


**Table 2.** Colour parameters and optical index of chitosan films modified with different percentages of EPPE.

Data are presented as mean <sup>±</sup> SD. Means with different superscripts (a–d) in lowercase letters in a column are significantly different at *p* < 0.05.

The reduced WI for Ch-EPPE films suggested that the film was beginning to become darker, allowing for the preservation of light-sensitive food items [36]. Opacity measurements revealed that the Ch-EPPE films were significantly (*p* < 0.05) impenetrable, with the Ch-5% EPPE film having a 73.24% increase in opacity over the control (Ch-0% EPPE). Because the chitosan polymer backbone has a mostly linear structure and offers the least resistance to light penetration compared to globular plasticizers, a comparison of the WI and opacity values of all the Ch-EPPE films with the control reveals a small quantitative drop in whiteness and a small rise in opacity [37]. Siripatrawan and Harte [29] obtained similar results.
